According to Joseph Conrad, "It is not the clear-sighted who rule the world. Great achievements are accomplished in a blessed, warm mental fog." It's a statement I've always found reassuring, my own mind being in such a permanent muddle that, were it a computer, I'd be sorely tempted to wipe the hard drive and start all over again.

The problem is not one of forming an opinion; in my experience, they are remarkably easy to form. The challenge lies in completing the jigsaw, achieving some sort of composite picture from the mass of competing, and at times contradictory, certainties.

I've been thinking about this a lot recently in connection with the debate on assisted suicide. While it strikes me that the choice of a rational adult to end their life must be theirs by right, and that the threat of prosecution for people who assist those who cannot act alone contravenes that right, the consequences of changing the law are potentially devastating, not only for those with mental health problems but for the mental health of society as a whole.

This is not an easy subject to discuss. It's a topic about which people feel strongly, and understandably so. Moreover, there is sometimes a sense that if you aren't suffering from a terminal illness, or caring for someone with terminal illness, or profoundly disabled, then you have no right to express a view at all. But the law on assisted suicide, and indeed on euthanasia, has the greatest possible impact on our attitude towards life itself, and no one is exempt from its implications. It is crucial that the psychological signal of any proposal to change the law be properly evaluated.

An argument frequently made in favour of legalising assisted dying is that people should have the freedom to choose for themselves. As long as there are adequate measures to protect "the vulnerable", as long as no one is being leant on, then what have we got to lose? What we have to lose, I would suggest, is the choice not to have a choice.

Any proposal containing a clause excluding "the vulnerable" arouses my instant suspicions. There may be valid reasons for such a clause, but I can't think of any. Or, more precisely, what I can't think of is a human being who isn't vulnerable – which renders such safeguards misleading at best. The idea that people with something to gain might pressurise others to end their lives is certainly a concern, but such external pressure is as nothing compared with the pressures people place upon themselves.

It is this that makes meaningless any attempt to equate the euthanising of animals with that of human beings. Animals suffer, certainly, but they lack the level of consciousness required for self-reflection. In other words, they do not torture themselves. People, on the other hand, are highly adept at self-torture. As Conrad put it, "What makes mankind tragic is not that they are the victims of nature, it is that they are conscious of it."

If we legalise euthanasia, we inevitably shift our attitude towards suffering. There's a sense in which pain becomes self-imposed, and even self-indulgent. Not only does this have profound implications for our willingness to fund palliative care and to provide for people who are disabled, but also for our perception of difference in general. The danger is that, as a society, we become less tolerant. We become a society in which it is easy to imagine people convincing themselves they have become a burden, that they have no right to impose on others the pain of watching their suffering – that, ultimately, they have no right to go on.

The director of public prosecutions, Keir Starmer, has performed an impressive feat. In issuing his interim guidance on factors to be considered when deciding whether to prosecute the offence of assisted suicide, he illuminates just enough of the path to reassure those who need help to die, while keeping the edges sufficiently foggy to allow for discretion in each individual case. It is a human and humane approach to a massively complex issue. My fear is that it won't be enough to satisfy those who want the whole thing floodlit. My earnest plea is that we strive to embrace the fog.